Hitherto, the photorefractive effect has been found with second-order non-linear optical crystals of lithium niobate, barium titanate, strontium barium niobate, bismuth silicon oxide, etc. and investigated and utilized as a space light modulator, real time hologram recording device and phase conjugate device.
Organic second-order (quadratic) non-linear optical materials have recently been attracting attention because of their expected high non-linear optical constant. Extensive researches and investigations are being conducted particularly for the purposes of their use for wavelength conversion of laser beams and as an electro-optical modulator based on the Pockels effect.
Further, there is a recent new confirmation that a photorefractive effect, which has heretofore been confirmed only with inorganic crystals, is also provided by organic crystals, which comprise 2-cyclooctyl amino-5-nitropyridine doped with tetracyanoquinoline (TCNQ) and serve as second-order non-linear optical materials (K. Sutter, J. Hulliger and P. Guunter, Solid State Communications, Vol. 74, No. 8, page 867-870, 1990).
Further, with a polymer composition, which is obtained by doping an epoxy polymer composed of 2,2-bis(4'-hydroxyphenyl)propane diglycidylether (i.e., commonly termed diglycidylether of bisphenol A) and 4,nitro-1,2-phenylenediamine with diethylaminobenzaldehydrazone, a photoconductive material, having a glass transition temperature of 65.degree. C. and which generates diffracted light on the order of 0.001%, is observed to exposing the composition by holographic exposure at a light intensity of 13 W/cm.sup.2 in the presence of an applied electric field to promote the orientation of the nitroaniline part (Stephan Ducharme, J. C. Scott, R. J. Tweig and W. E. Moerner, Physical Review Letters, 65(14), 1846-1849, 1991).
However, with inorganic materials such as bismuth silicon oxide, large single crystals can not be readily obtained, and thus it is difficult to produce a hologram recording device having a large recordable area. The device is thus expensive because of a low efficiency of manufacture.
With organic materials, on the other hand, it is thought to be possible to increase the area and reduce the price of the hologram recording device. However, 2-cyclooctyl amino-5-nitropyridine doped with tetracyanoquinoline noted above, is infeasible because its sensitivity and diffraction efficiency are both low.
Besides, since it is an organic crystal, it is not easy to obtain a hologram recording device having a large area.
Further, with an epoxy-based polymer non-linear optical material doped with a photoconductive material of low molecular weight as noted above, although it is a polymer composition, the solubility of the photoconductive material is low, and the composition is incapable of thermal hardening in a polarized state. Stability thus is lacking. In addition, the composition is inferior in the charge generation capacity and has substantially no charge trap. Therefore, it is impossible to expect formation of a large space charge distribution. For example, it is impossible to expect increase in the diffraction efficiency.